Method for modifying the surface of a polymer membrane, and a membrane thus modified

ABSTRACT

The present invention concerns a method for modifying the transfer characteristics of a porous organic or inorganic membrane, in which 
     a) on the said membrane, at least one layer is formed by applying at least one homogeneous solution obtained by mixing one or more rare-earth or alkaline-earth fluoroalkoxides in an anhydrous organic solvent at room temperature and in an inert atmosphere, 
     b) hydrolysis is effected by placing the coating formed at a) in contact with a quantity of water at least equal to the stoichiometric quantity required to hydrolyse the fluoroalkoxides, 
     c) the membrane is rinsed with water to eliminate the soluble salts formed. The membranes thus modified are useful for the regeneration of photographic solutions, in particular black and white developers, or for the separation of organic compounds from aqueous effluents.

The present invention concerns a method for modifying the surface of amembrane and membranes which can be obtained using this method. Themethod according to the invention enables the surface of a porousorganic or inorganic membrane to be modified, so as to control thevolume of liquid transported and the diffusion of species through themembrane. The membranes thus modified are useful for the regeneration ofphotographic solutions, in particular black and white developers, or forthe separation of organic compounds from aqueous effluents.

Membranes are widely used in separation techniques. The transport offluids through membranes takes place by means of different mechanisms,which depend on the structure and nature of the membrane. The mostwidely used membranes are formed from synthetic or natural organicpolymers. Porous membranes contain voids which are large compared withthe size of the molecules transported. In these membranes, the pores areinterconnected and the solid materials represent only a small percentageof the total volume of the membrane. The porous membranes availablecommercially have a pore size of between 0.005 μm and 20 μm. They aremade from a great variety of polymers so as to obtain a wide range ofrigidities and mechanical strengths. Generally, for separation in theaqueous phase, either hydrophilic membranes or hydrophobic membranes areused, according to the experimental conditions (pH, oxidising medium),but also according to the type of molecules to be separated. Thusmolecules of the hydrophobic type will tend to be adsorbed more on ahydrophobic support. In order to prevent these molecules from beingadsorbed, the surface of the hydrophobic support can be modified byincorporating a hydrophilic group or by means of a fine surfacedeposition of a hydrophilic polymer.

For pressurised technologies (microfiltration, ultrafiltration,nanofiltration, reverse osmosis), polymers of the hydrophilic type arequite suitable, since they make it possible to have high flows. This istrue of cellulose and its derivatives, which have been used in reverseosmosis for a very long time (high flow, good separation selectivity,low cost). However, porous membranes of the hydrophilic type are highlysensitive to the phenomenon of swelling when they come into contact withan aqueous solution (and this behaviour can be more pronounced dependingon the pH). Furthermore, these hydrophilic materials have low stabilityin the presence of certain polar organic solvents, acids or bases, aswell as oxidants (chorine, hypochlorite). These materials are alsoextremely sensitive to bacterial growth, all the more so since theycannot be subjected to base+oxidant or acid+oxidant treatments owing totheir low resistance to these chemical agents. For these materials,control of the hydrophilic/hydrophobic balance enables the chemicalstability of the support to be increased as a function of the pH and thevarious aforementioned reagents. It also increases the resistance ofthese materials to the mechanical stresses arising from the pressure ofuse.

For systems of the dialysis type, in general what is involved is theseparation of the macromolecules and the mineral species (salts) ormolecules with a low molecular weight. In this case, hydrophilicmaterials of the cellulose type and derivatives of cellulose are verywell suited to treatment of such environments (high proportion ofwater). The osmotic forces constitute the "driving" element of thisseparation process. In this precise case, modification of thehydrophilic/hydrophobic balance will be used to increase the stabilityof the membrane vis-a-vis the experimental conditions (pH, presence oforganic solvents, etc). In general, in the case of dialysis, no attemptis made to control the flow of water; the initial solution is dilutedand the small molecules are transported through the membrane. If thesolution is to be concentrated, another technology such asultrafiltration is used in addition to dialysis. On the other hand, inthe case of regeneration of photographic baths, the control of the flowof water is an important parameter, since it is not wished to dilute thesolution to be treated.

Other asymmetrical polymer materials obtained from mixtures of monomersexist. Their functioning is described, for example, in Chapter I,entitled "Physical Chemistry of Membranes", page 19 of Membrane Scienceand Technology, edited by Y. Osada and T. Nakagawa. "The hydrophobicdomain prevails on one side of the membrane, where in contact with thehydrophobic substrate, and the hydrophilic domain prevails on the otherside of the membrane. A flow reversal effect has been observed for suchasymmetric membranes when the concentration dependence of the diffusioncoefficient through a hydrophilic membrane is marked. A highpermeability coefficient is obtained when the hydrophilic penetrantpermeates the membrane from the hydrophilic side of the asymmetricmembrane. On the other hand, the permeability coefficient is low whenthe hydrophilic penetrant permeates from the hydrophobic domain side".

Hydrophobic porous membranes are highly resistant to chemical substancesand do not swell in water. On the other hand, they function only underpressure, and even under these conditions they do not allow the water topass sufficiently. It is therefore necessary to treat these membranes inorder for their pores to have a hydrophilic surface. Numerous knownmethods for making the surface of hydrophobic membranes hydrophilic aredescribed in "Synthetic Polymeric Membranes, a Structural Perspective",Second Edition, by Robert E Kesting, published by Wiley-Interscience(New York, 1985).

For example, U.S. Pat. No. 5,098,569 describes a membrane support with amodified surface, in which a monomolecular layer of a hydrophilicpolymer derived from cellulose is grafted onto a porous hydrophobicmembrane. The membrane obtained is stable in ethanol.

Polyacrylonitrile membranes are naturally rather hydrophobic but are notlipophobic. For certain specific applications, it is necessary toincrease their lipophobia so as to avoid clogging by organic compounds.They are electrostatically neutral and possess higher physicalresistance to alkalis than cellulose and its derivatives. Functionalisedpolyacrylonitrile membranes are available commercially which are usedfor the recycling of paint baths, the purification and concentration ofanimal or vegetable proteins and the purification of vaccines andantibodies. The use of these membranes under pressure enables theosmotic pressure forces to be compensated for, but if they are used in aprocess of the dialysis type, a phenomenon of osmosis with passage ofthe dilute phase into the concentrated phase is observed, the size ofthe pores being the sole selection factor in the particular case ofdialysis. (In the normal case where they are used with a pressuregradient in ultrafiltration, the electrostatic effects associated withthe surface charge are also very significant).

Modification of the surface of a membrane having a hydrophilic characteris also known. For example, the patent DD 296220 describes a method formodifying the surface of cellulose membranes by causing a cellulosemembrane to react with a carboxylic acid halide in solution in anaprotic polar solvent, in a medium catalysed by a base. The halogenatedfunction is thereafter partially sulphonated with bisulphite, and thenthe membrane is rinsed. These membranes are used in dialysis and arecompatible with blood.

The patent DD 278495 describes a method of obtaining a haemodialysismembrane by modifying the surface of a regenerated cellulose membranemodified by treatment with a polyisocyanate. The membranes modified inthis way can be used in haemodialysis, haemofiltration or diafiltration.These membranes, like those of the patent DD 296220, are not designedfor the separation of organic compounds and salts in aqueous media.

The regeneration of photographic baths generally is effected withion-exchange resins, in the case of developers of the colour type, or byelectrodialysis. In the case of black and white developers, it is wishedto keep the activity in the bath constant, for example throughcontinuous selective elimination of the bromide ions in a developingbath. No known technique enables the halides to be separated from theorganic compounds, such as hydroquinone and its derivatives. Until now,it was not possible to use dialysis for the regeneration of black andwhite baths, since the known membranes did not enable the flow of waterto be controlled to avoid dilution of the bath.

The method according to the present invention makes it possible tomodify the transfer characteristics of a porous organic or inorganicmembrane, that is to say to reduce the flow of liquid transported, andto control the diffusion of the species in solution through themembrane, thereby improving the efficiency of the separation.

The method according to the invention comprises the following steps:

a) on the said membrane, at least one layer is formed, obtained byapplying at least one homogeneous solution obtained by mixing one ormore rare-earth or alkaline-earth fluoroalkoxides in an anhydrousorganic solvent at room temperature and in an inert atmosphere,

b) hydrolysis of the fluoroalkoxide or fluoroalkoxides is effected byplacing the layer formed at a) in contact with a quantity of water atleast equal to the stoichiometric quantity required to hydrolyse thefluoroalkoxides,

c) the membrane is rinsed with water to eliminate the soluble saltsformed.

Another object of the invention concerns the novel membranes obtainableby this method.

Another object of the invention is the use of these membranes for theregeneration of photographic solutions, in particular black and whitedevelopers, and the separation of organic compounds from aqueouseffluents.

The method according to the present invention makes it possible tomodify selectively, by increasing its hydrophobic and/or lipophobiccharacter, either solely the surface of the pores within the membrane,or the entire surface of the porous membrane (that is to say all theexternal surfaces of the membrane and the surface of the pores which aredistributed within the membrane). As will be seen in the examples, thetype of modification made will depend on:

1) the polymer material of the membrane. (Hydrophilic cellulose,possessing free hydroxyl groups, allows higher reactivity of thefluoroalkoxides, while a material of the "hydrophobic type" will havelower reactivity with fluoroalkoxides).

2) the alkaline-earth (or rare-earth) metal associated with thefluoroalkoxide,

3) the alkoxo radical. The preponderant factors are the length of thecarbon chain, the branched or linear character of the chain and thenumber of fluorine atoms.

BRIEF DESCRIPTION OF DRAWINGS

In the description that follows, reference will be made to the drawingsin which:

FIG. 1 depicts an assembly used for the regeneration of photographicbaths,

FIG. 2 depicts a diagrammatic view in cross section of a cellulosemembrane, and

FIG. 3 depicts a diagrammatic view in cross section of apolyacrylonitrile membrane.

DETAILED DESCRIPTION

In step a), the alkaline earths are chosen from amongst elements ingroup IIA, such as beryllium, magnesium, calcium, strontium, barium orradium, and the rare earths are chosen from amongst elements in groupIIIB, such as scandium, yttrium, lanthanum, cerium, gadolinium, erbiumor ytterbium. The preferred fluoroalkoxides are barium, calcium andstrontium fluoroalkoxides.

The initial rare-earth or alkaline-earth fluoroalkoxides can be obtainedby any one of the methods known in the art. In general, they areobtained from the corresponding alkaline-earth or rare-earth alkoxide oralkoxides.

The preparation of alkaline-earth alkoxides can be effected by varioussyntheses known in the art. A synthesis which is easy to implementconsists of reacting an alcohol directly on an alkaline earth. The yieldof such a synthesis depends in particular on the steric hindrance of thealcohol used. The lower the steric hindrance of the alcohol, the fasterthe synthesis of the alkaline-earth alkoxides. Preferably, the alcoholis chosen from amongst methanol, ethanol or propanol.

Rare-earth alkoxides can be synthesised either by reacting a rare-earthchloride with an alkaline-metal alkoxide, or reacting rare earthdirectly with an alcohol such as 2-propanol, or a functionalised alcoholsuch as 2-methoxyethanol or ethylene glycol monomethyl ether.

The rare-earth or alkaline-earth fluoroalkoxides used in the presentinvention can be obtained either through the alcoholysis of rare-earthor alkaline-earth alkoxides by a fluorinated or perfluorinated alcohol,hereinafter referred to as a "fluoroalcohol", or by reacting thealkaline earth with the fluoroalcohol directly in solution in a polarsolvent. Preferably, they are obtained by the alcoholysis of barium,strontium and calcium alkoxides with fluoroalcohols.

Fluoroalcohols are hydrogenated alcohols with a more or less long chain,straight or branched. The fluoroalcohols able to be used in the presentinvention have at least 3 and at most 10 fluorine atoms and a fluorineto carbon ratio of at least 1.5 and at most 2.5. The preferredfluoroalcohols have a number of fluorine atoms between 3 and 10 and anumber of carbon atoms between 2 and 5 and are, for example, chosen fromamongst perfluorotertiobutanol, 2,2,2-trifluoroethanol,1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,2,3,4,4,4-hexafluoro-1-butanol,1,1,1,2,2,3,3-heptafluoro-4-butanol or2,2,3,3,4,4,5,5-octafluoro-1-pentanol.

In step a) of the method of the present invention, on the porous supportforming the membrane, a solution is applied which contains thefluoroalkoxide or fluoroalkoxides in an anhydrous organic solventmiscible with water, in an inert atmosphere and at room temperature. Thepreferred solvents are tetrahydrofuran, alcohols or ketones. Thequantity of solid matter in the solution containing the fluoroalkoxideis between 0.1 and 100 g/l.

The porous supports which can be used within the scope of the presentinvention can be mineral membranes or organic polymer membranes, such asthose described in Reverse osmosis and ultrafiltration, Chapter II,entitled "Technology and applications", by Alain Maurel, in Techniquesde l'ingenieur J2796, pages 4 to 13.

The mineral membranes are chosen from amongst membranes made of silica,alumina, zirconia or titanium oxide or mixtures of these oxides or elsecarbon membranes, optionally coated with a fine layer of oxide.

The organic polymer membranes are, for example, membranes of celluloseor cellulose derivatives, preferably regenerated cellulose, or membranesof polyacrylonitrile, polysulphone, polyethersulphone orfluorocarbonated polymers. The preferred organic membranes have reactivegroups on the surface, for example hydroxyl groups as in cellulosesupports.

Organic or mineral membranes can include functionalised groups making itpossible to have a positive surface charge (for example by introducingammonium or phosphonium groups), or to have a negative surface charge(for example by introducing sulpho, carboxy groups, etc).

The membranes can take the form of flat, spiral or tubular modules, orof hollow fibre modules.

It is possible to apply the fluoroalkoxide solution to the surface ofthe membrane by any known method for applying sol-gel, for example usinga coating bar, an air knife by roll coating or by soaking, spin coating,bead coating, curtain coating or by spraying or by circulating themetallic fluoroalkoxide in the reactor containing the membrane underappropriate conditions.

It has been found that, if the membrane is moistened prior to step a),the extraction and separation performance of the membrane is improved.The moistening of the membrane affords cleaning and activation of thesurface of the membrane, and facilitates the hydrolysis reaction.Moistening is obtained by soaking in a solution of water and alcohol orby placing the membrane in an atmosphere saturated with water vapour.

In step b), the quantity of water for hydrolysing the fluoroalkoxidesmust be at least equal to the stoichiometric quantity and less than 5times, preferably less than twice, this stoichiometric quantity. Ingeneral, the process is carried out at atmospheric humidity. It ispossible to combine this hydrolysis with any other known complementarymethod that does not impair the organic support, such as placing themembrane in an oven under controlled humidity.

After hydrolysis, the solvent is left to evaporate and a hydrophobiclayer based on alkaline earth (or rare earth) is obtained.

It is possible to vary the thickness of the coating obtained, either byvarying the initial concentration of metallic fluoroalkoxides or byrepeating the sequence a)-b) several times and leaving the poroussupport in the open air for several minutes between each deposition. Itis also possible to produce successively, in the same way, severallayers with fluoroalkoxides differing in terms of the nature of thealkoxo radical or in terms of the nature of the alkaline-earth (orrare-earth) metal.

At step c), the membrane is rinsed with water. This step eliminates thewater-soluble metal salts which could be detrimental when the membraneis used to treat certain solutions, such as photographic solutions.

EXAMPLES A-H

These examples concern the preparation of the fluoroalkoxides used inthe present invention.

Example A

Synthesis of barium fluoroalkoxide Ba6R

Under argon, 13.6 g (0.0099 mol) of barium is added to 200 ml ofanhydrous ethanol to form a solution. The reaction is exothermic, withthe release of hydrogen. The reaction medium is filtered in order toeliminate residual colloids. The filtrate is concentrated at 10⁻² mm Hgand dried for 12 hrs to give a dusty white powder, elementary analysisof which shows that it contains approximately 60% barium by weight.

Under argon, 22.5 g of this powder is introduced into 300 ml ofanhydrous tetrahydrofuran (THF), and then 21 ml of hexafluoro-2-propanolis added drop by drop at room temperature.

The mixture is left to react for 2 hours under agitation (highlyexothermic reaction). The product is purified by crystallisation inanhydrous tetrahydrofuran. 42 g of white powder is recovered, elementaryanalysis of which shows that it contains approximately 30% barium byweight.

Example B

Synthesis of barium fluoralkoxide Ba6L

1.8 ml of hexafluorobutanol is added to a mixture consisting of 1.7 g ofwhite powder produced by the action of anhydrous ethanol on the bariumprepared as in Example A, and 100 ml of a 1:1 mixture of anhydrous THFand anhydrous ethanol, under argon at room temperature.

The solution turns orange and the reaction is slightly exothermic.Little by little, the formation of particles in suspension is observed.By leaving it to decant, 3.76 g of fine powder is obtained, elementaryanalysis of which shows that it contains approximately 28% barium byweight.

Example C

Synthesis of calcium fluoroalkoxide Ca6R

Under argon, 29.7 g (0.72 mol) of calcium is added to 200 ml ofanhydrous ethanol to form a solution. The reaction is catalysed withhexamethylsilazane and the solvent is brought to reflux. Amicrocrystalline white powder precipitates when the medium is cooled,and for this reason the reaction medium is filtered hot. The filtrate isevaporated and dried at 10⁻² mm Hg. A fine powder is recovered,elementary analysis of which shows that it contains approximately 30%calcium by weight.

Under argon, 11.2 g of the white powder produced by the reaction of theanhydrous ethanol on the calcium is introduced into 100 ml of anhydroustetrahydrofuran (THF), and then 20 ml of hexafluoro-2-propanol is addeddrop by drop at room temperature.

The mixture is left to react for 4 hours under agitation (highlyexothermic reaction). The clear reaction medium is evaporated dry anddried for 12 hrs at 10⁻² mm Hg. 24.5 g of dusty white powder isrecovered, elementary analysis of which shows that it containsapproximately 10% calcium by weight.

Example D

Synthesis of barium fluoroalkoxide Ba3L

The procedure of example A is followed, replacing thehexafluoro-2-propanol with trifluoroethanol. When fluoroalcohol isadded, the reaction is slightly exothermic. After a day under agitationat room temperature, the clear reaction medium is evaporated dry anddried for 12 hrs at 10⁻² mm Hg. A dusty powder is obtained, elementaryanalysis of which shows that it contains approximately 70% barium byweight.

Example E

Synthesis of barium fluoroalkoxide Ba7L

The procedure of example A is followed, replacing thehexafluoro-2-propanol with 4-heptafluoro-1-butanol. When fluoroalcoholis added, the reaction is slightly exothermic. After evaporation anddrying, 3.45 g of dusty powder is obtained, elementary analysis of whichshows that it contains approximately 26% barium by weight.

Example F

Synthesis of barium fluoroalkoxide Ba8L

The procedure of example A is followed, replacing thehexafluoro-2-propanol with octafluoro-1-pentanol. When fluoroalcohol isadded, the reaction is not exothermic. The clear reaction medium isevaporated dry and dried for 12 hrs at 10⁻² mm Hg. 5.75 g of a finepowder is obtained, elementary analysis of which shows that it contains23% barium by weight.

Example G

Synthesis of calcium fluoroalkoxide Ca8L

The procedure of example C is followed, replacing thehexafluoro-2-propanol with octafluoro-1-pentanol. When fluoroalcohol isadded, the precipitation of a white substance is observed. The substanceis isolated by filtration and then dried under vacuum for 12 hrs at 10⁻²mm Hg. A viscous white substance is obtained, elementary analysis ofwhich shows that it contains approximately 8% calcium by weight.

Example H

Synthesis of strontium fluoroalkoxide Sr6R

9.9 g of strontium (0.113 mole) is introduced into 100 ml of anhydrousethanol. The reaction is exothermic. After 12 hrs of heating of theethanol at boiling point, the reaction medium is filtered hot. As soonas the filtrate cools to room temperature, a crystalline white substanceprecipitates. This substance is isolated by filtration and dried for 12hrs at 10⁻² mm Hg. 18 g of a white powder is obtained, elementaryanalysis of which shows that it contains approximately 50% strontium byweight.

This white powder is put in suspension in anhydrous THF, then 22.5 ml ofhexafluoroisopropanol is added, and the medium clears. The reactionmedium is then evaporated dry and dried for 12 hrs at 10⁻² mm Hg. Adusty yellow powder is obtained, elementary analysis of which shows thatit contains approximately 21% strontium by weight.

EXAMPLES 1-4

These examples describe how a hydrophobic layer is obtained on a poroussupport.

In these examples, the various fluoroalkoxides are deposited on a porouscellulose or polyacrylonitrile support. The fluoroalkoxides are put insolution at 1 g of fluoroalkoxide to 50 ml of anhydrous ethanol underinert gas. Four passes are effected over the support by means of a bar,depositing 125 μm of solution each time. Between each deposition, theporous support is left in the open air for 5 mins. Hydrolysis takesplace with the moisture present in the air. The solvent is left toevaporate. A hydrophobic layer based on alkaline earth is obtained.

After 5 mins, the porous support is immersed in a beaker of waterosmosed to eliminate the soluble salts of Ca, Ba or Sr formed.

To evaluate the change in the surface state, the wetting angle of theporous support is measured

before the layer is deposited,

after the layer obtained by the method described above is deposited,

after the use of the membrane in a bath of photographic developercontaining silver halides and organic compounds used to developradiographic products.

The wetting angle θ is determined by the Wilhemy strip method, which isbased on the measurement of the force required to pull from a liquid athin sheet of a sample suspended on one of the arms of a balance andimmersed in this liquid. The liquid is maintained at 24° C. The surfacetension of the liquid γ is first measured using a strip of filter paperfor which θ=0. The wetting angle is defined by the following formula

    cos θ=ΔW/Peγ

where

ΔW is the variation in the weight of the sheet at the moment it makescontact with the liquid, and

Pe is the perimeter of the sheet.

Within the scope of the present invention, a variation in the wettingangle of ±3° shows a change in the surface state. The hydrophobiccharacter increases with the value of the wetting angle.

The measurement of the "treated side" wetting angle shows the change inthe surface state of the membrane on the side where the fluoroalkoxidesolution is applied.

The measurement of the "untreated side" wetting angle shows the changein the surface state of the membrane on the side opposite the side wherethe fluoroalkoxide solution is applied.

The measurement of the "used side" wetting angle shows the change in thesurface state of the membrane when it has been placed in contact withthe developer for several hours. A small reduction in the wetting angleshows that the hydrophobic layer obtained by the method of the inventionis stable in a highly basic medium.

In order to determine the separation characteristics of the organicmolecules and salts, the membrane is placed in contact with a developingsolution with the formula:

    ______________________________________                                        hydroquinone (HQ)        21.00 g/l                                            hydroquinone monosulphate (KHQS)                                                                       13.40 g/l                                            phenidone-A              0.69 g/l                                             bromide*                 3.46 g/l                                             sulphite*                8 g/l                                                water qsp 1l                                                                  ______________________________________                                         *The bromide and sulphite are in the form of Na salts.                   

In order to test the membranes, the assembly depicted in FIG. 1 is used.The system which comprises the membrane is composed of two compartments5) and 6) each containing approximately 50 ml, separated by a membrane7) of 10 cm², the whole forming a sealed system. 500 ml of developingsolution contained in the reactor 3) circulates in the compartment 5)from the inlet 9) to the outlet 8) by means of a pump 1) with a flowrate of 25 ml/min. 180 ml of water contained in the reactor 4)circulates in the reverse direction in the compartment 6) from the inlet11) to the outlet 10) by means of the pump 2) with a flow rate of 25ml/min.

Example 1

Cellulose membrane

This example concerns the modification of the surface state ofSpectra/Por regenerated cellulose membranes having a cutoff threshold of6000 to 8000 daltons (diameter of pores around 2 nm) when they aretreated with various fluoroalkoxides.

                  TABLE 1                                                         ______________________________________                                                    Wetting angle                                                     Fluoroalkoxide                                                                              Untreated side                                                                           Treated side                                         ______________________________________                                        none          34         38                                                   Ca6R          75         73                                                   Ba6R          54         117                                                  ______________________________________                                    

A significant change is observed in the hydrophobia of the cellulosesupport treated with the two fluoroalkoxides, compared with theuntreated control. The barium fluoroalkoxide reacts rapidly, and themembrane treated in this case exhibits high asymmetry, as seen in thedifference between the values of the wetting angle of the treatedside 1) and untreated side 3) depicted in FIG. 2. With bariumfluoroalkoxide, the side 1) is treated preferentially, the surface ofthe pores 2) and the side 3) also being treated, but to a lesser degree.With calcium fluoroalkoxide, hydrolysis is slower and both sides of themembrane 1) and 3) and the surface of the pores 2) exhibit the samehydrophobia.

Example 2

Selectivity of cellulose membranes

This example shows the improvement obtained in terms of flow andselectivity with the cellulose membrane of the preceding example treatedwith Ba6R.

                  TABLE 2                                                         ______________________________________                                                               % HQ + KHQ    % pheni-                                 Fluoro % H.sub.2 O/                                                                          % Br/   S/            done-                                    alkoxide                                                                             24 hrs  24 hrs  24 hrs   FS1  A/24 hrs                                                                             FS2                               ______________________________________                                        none   15      26      24       1.1  32     0.8                               Ba6R    2      16       3       5.3   8     2.0                               ______________________________________                                    

%H₂ O/24 hrs represents the flow of water, that is to say the volume ofwater transferred into the developer in 24 hrs:

%H₂ O/24 hrs=100(Vf_(rev) -Vinit_(rev))/Vinit_(rev)

where

Vf_(rev) represents the final volume of developer after 24 hrs oftreatment through the membrane,

Vinit_(rev) represents the initial volume of the developer.

%Br/24 hrs represents the % of bromide extracted from the developingsolution in 24 hrs.

%HQ+KHQS/24 hrs represents the % by weight of hydroquinone andhydroquinone monosulphate extracted from the developing solution in 24hrs.

%phenidone-A/24 hrs represents the % of phenidone-A extracted from thedeveloping solution in 24 hrs.

FS1 is the selectivity factor of Br with respect to HQ+KHQS (ratio as a% extracted in 24 hrs).

FS2 is the selectivity factor of Br with respect to phenidone-A (ratioas a % extracted in 24 hrs).

Given the low porosity of cellulose, ion transfers are slow, and this isthe reason why the values are given for 24 hrs. The quantity of bromideextracted with the membrane treated with Ba6R is lower but the flow ofwater is considerably reduced and the selectivity factors significantlyimproved.

Example 3

This example concerns the modification of the surface state ofpolyacrylonitrile membranes of the IRIS A -40000D type, as referenced inthe Prolabo 1996-1997 catalogue, when they are treated with variousfluoroalkoxides.

                  TABLE 3                                                         ______________________________________                                                 Wetting angle                                                        Fluoroalkoxide                                                                           Untreated side                                                                             Treated side                                                                            Used side                                   ______________________________________                                        none       61           67        47                                          Ba3L       60           74        59                                          Ba6L       82           80        66                                          Ba6R       85           95        86                                          Sr6R       58           77        68                                          Ca6R       63           67        59                                          Ba7L       81           77        66                                          Ba8L       71           74        71                                          Ca8L       71           86        73                                          ______________________________________                                    

As the porosity of the polyacrylonitrile membranes is less regular thanthat of the cellulose membranes, and the reactivity of these membraneswith the fluoroalkoxides is lower than that of the cellulose, thefluoroalkoxides diffuse differently. The polyacrylonitrile membranetreated with Ba6R has less pronounced asymmetry than the cellulosemembrane treated with the same fluoroalkoxide.

Whatever the fluoroalkoxide, the treated side exhibits greaterhydrophobia after use than the control membrane, which proves that thepolyacrylonitrile membrane has been modified. The change in the wettingangle is at its maximum for a branched alkoxo radical containing 6fluorine atoms (Ba6R).

According to the values of the wetting angles,

1) comparing Ba6R, Sr6R and Ca6R, it is seen that, with the same alkoxoradical, barium is more efficacious than strontium, which is itself moreefficacious than calcium,

2) comparing Ba6R and Ba6L, it is seen that, for the same number offluorine atoms, the branched alkoxo radical is more efficacious than thelinear radical,

3) comparing the linear alkoxo radicals for the same alkaline earth, itis seen that the chain is too short (Ba3L), and there is no change inthe wetting angle. If the chain is long (Ba8L or Ca8L), the reactivityof the fluoroalkoxide falls and the change in the wetting angle is lessthan with Ba6L.

The wetting angle decreases a great deal for the control membrane afteruse. This shows that the control membrane cannot be used for continuoustreatment. On the other hand, for treated membranes, the wetting angleremains larger than that of the control membrane before use. This showsthat the treated membranes are stable in an alkaline environment andresistant to organic compounds.

The most hydrophobic character is that obtained with Ba6R. A slight fallin the value of the wetting angle is observed after the membrane hasbeen placed in contact with the developing solution for two 24-hourcycles.

As in the case of cellulose membranes, for treatment with Ca6R, noasymmetry is observed in the treatment of the two sides and furthermorethe hydrophobia of the surface is unchanged. However, the measurement ofthe wetting angle on the used side shows that the membrane is morestable than the control in an alkaline environment.

It is probable that the difference in behaviour between cellulose andpolyacrylonitrile is due to the fact that the hydroxyl groups incellulose assist the in-situ hydrolysis reaction of the fluoroalkoxide.

Example 4

Selectivity of polyacrylonitrile membranes

This example shows the improvement obtained in terms of flow andselectivity with the polyacrylonitrile membranes in the precedingexample.

                                      TABLE 4                                     __________________________________________________________________________                                 % pheni-                                         Fluoro                                                                             % H.sub.2 O/                                                                      % Br/                                                                             % Br/                                                                             % HQ + KHQS done                                             alkoxide                                                                           24 hrs                                                                            6 hrs                                                                             24 hrs                                                                            6 hrs   FS1 A/6 hrs                                                                            FS2                                         __________________________________________________________________________    none 36* 17  25  20      0.85                                                                              11   1.5                                         Ba3L 36**                                                                              --  --  --      --  --   --                                          Ba6L  3  16  35.8                                                                              5.1     3.13                                                                              12.8 1.2                                         Ba6R 18  22  34  7.5     2.93                                                                              12.5 1.8                                         Sr6R  3  20.8                                                                              25  9       2.31                                                                              10.3 2.0                                         Ca6R 14  40  28  4.1     9.75                                                                              12.3 3.2                                         Ba7L 43  28  30  4       7   13   2.1                                         Ba8L 10  32.8                                                                              37  3.5     9.37                                                                              13.3 2.5                                         Ca8L  2  29.6                                                                              31.4                                                                              12.1    2.44                                                                              16   1.8                                         __________________________________________________________________________     *for the purpose of this experiment, after 24 hrs of operation, all the       water had diffused into the compartment containing the developer, and         after 6 hrs of operation 90% of the water had diffused into this              compartment.                                                                  **For the purpose of this experiment, all the water had diffused into the     compartment containing the developer after 1 hr of operation. Excessive       transport of water rendered extraction measurements impossible.          

*for the purpose of this experiment, after 24 hrs of operation, all thewater had diffused into the compartment containing the developer, andafter 6 hrs of operation 90% of the water had diffused into thiscompartment.

**For the purpose of this experiment, all the water had diffused intothe compartment containing the developer after 1 hr of operation.Excessive transport of water rendered extraction measurementsimpossible.

%H₂ O/24 hrs is as defined previously.

%Br/6 hrs and %Br/24 hrs represent the % of bromide extracted from thedeveloping solution, in 6 hrs and 24 hrs respectively.

%HQ+KHQS/6 hrs represents the % by weight of hydroquinone andhydroquinone monosulphate extracted from the developing solution in 6hrs.

%phenidone-A/6 hrs represents the % of phenidone-A extracted from thedeveloping solution in 6 hrs.

FS1 is the selectivity factor of Br with respect to HQ+KHQS (ratio as a% extracted in 6 hrs).

FS2 is the selectivity factor of Br with respect to phenidone-A (ratioas a % extracted in 6 hrs).

Given the high porosity of the polyacrylonitrile membrane, ion transfersare rapid; this is why the extraction and extraction selectivity valuesare given at 6 hrs.

It can be seen that, compared with the control, better results areobtained for the flow of water, the % of bromide extracted and theselectivity with all fluoroalkoxides, except Ba3L, which does not formpart of the invention.

In the case of Ca6R, although the values of the wetting angle on thetreated and non-treated sides are very close to those of the control,comparisons between the flow and extraction values and those of thecontrol show that the membrane has been modified considerably by thetreatment.

Example 5

This example shows the improvement obtained in terms of flow andselectivity when the polyacrylonitrile membrane is moistened before theapplication of fluoroalkoxide. For this purpose, comparison is madebetween the performance of

a dry membrane, that is to say one washed with a mixture of water andethanol (1:1 by volume), dried for 24 hrs at room temperature and thentreated with Ba6R, and

a moistened membrane, that is to say one washed before step a) of thetreatment with a mixture of water and ethanol (1:1 by volume) and thentreated after 5 min with Ba6R.

                  TABLE 5                                                         ______________________________________                                                                    % HQ +     % pheni-                                      % H.sub.2 O/                                                                          % Br/  % Br/ KHQS/      done-                                  Membrane                                                                             24 hrs  6 hrs  24 hrs                                                                              6 hrs FS1  A/6 hrs                                                                              FS2                             ______________________________________                                        dry    18      16.7   33.8  5.3   3.1  8.3    2.0                             moist  30      35     32    0     ∞                                                                            5.8    6.0                             ______________________________________                                    

It can be seen that, when the membrane is moistened before theapplication of the fluoroalkoxide, the separation performance of themembrane is very much improved since the organic compounds are no longertransported but the flow of liquid increases.

We claim:
 1. Method for modifying the transfer characteristics of aporous organic or inorganic membrane, comprising the steps ofa) formingon said membrane, at least one layer from an homogeneous solutionobtained by mixing one or more rare-earth or alkaline-earthfluoroalkoxides in an anhydrous organic solvent at room temperatureunder inert atmosphere, b) hydrolysing the fluoroalkoxide orfluoroalkoxides by contacting the layer formed at a) with a quantity ofwater at least equal to the stoichiometric quantity required tohydrolyse the fluoroalkoxides, c) washing the membrane with water toeliminate the soluble salts formed.
 2. Method according to claim 1, inwhich the rare-earth or alkaline-earth fluoroalkoxide is the substanceobtained through alcoholysis of a rare-earth or alkaline-earth alkoxidewith a fluoroalcohol having at least 3 and at most 10 fluorine atoms anda fluorine to carbon ratio of at least 1.5 and at most 2.5.
 3. Methodaccording to claim 2, in which the alkaline earth is chosen from amongstBa, Ca and Sr.
 4. Method according to claim 3, in which thefluoroalkoxide is the substance obtained through alcoholysis of abarium, strontium or calcium alkoxide with a fluoroalcohol chosen fromamongst perfluorotertiobutanol, 2,2,2-trifluoroethanol,1,1,1,3,3,3-hexafluoro-2-propanol, 2,2,3,4,4,4-hexafluoro-1-butanol,1,1,1,2,2,3,3-heptafluoro-4-butanol or2,2,3,3,4,4,5,5-octafluoro-1-pentanol.
 5. Method according to claim 1,in which the membrane is a polymer membrane chosen from amongstmembranes of cellulose, polyacrylonitrile, polysulphone orpolyethersulphone, which can include functionalised groups introducing apositive or negative charge.
 6. Method according to claim 1, in whichthe organic solvent is chosen from amongst tetrahydrofuran, alcohols orketones.
 7. Method according to claim 1, in which the membrane ismoistened before application.
 8. Membrane obtainable by the method ofany one of claims 1,2,3,4,5,6, or
 7. 9. Method for the regeneration ofphotographic baths which comprises the step of contacting the bath withthe membrane made by the method of claim 1 and recycling as photographicbath the retentate obtained from the contacting step.
 10. Methodaccording to claim 9 wherein said photographic bath is a black and whitedeveloping bath.